The present invention relates to an anti-skid control system for a brake system in a vehicle, such as an automotive vehicle. More specifically, the invention relates to an anti-skid control system and a control method thereof, which can prevent a vehicle body from falling into spinning state due to difference of friction coefficient of left and right wheels relative to the road surface, and so forth.
Conventional anti-skid control system includes a valve for decreasing or holding hydraulic braking pressure on a road surface having low friction coefficient, particularly on a snowy road, icy road or so forth, for preventing wheels from locking and whereby assure stability during braking without causing expansion of braking distance.
However, such conventional system is not effective for assuring driving stability and braking efficiency at possible all road surface condition. Particularly, if there is a difference of the friction coefficients of the road surface at the left and the right wheels of the vehicle, and if control is performed to maintain the wheel slippage at the left and right wheels uniform, a difference of magnitude of the braking force applied to the left and right wheels is caused to generate yawing moment on the vehicle due to unbalance of the braking force.
In the conventional anti-skid control system, on the road surface condition having different friction coefficients, i.e. split .mu. road, select-low is performed to make the braking pressures coincidence with the braking pressure of one of the rear wheels which is going to cause locking, in order to obtain enhanced stability. In such select-low control, braking pressure becomes uniform at relatively low pressure level for all wheels, possibly too small, and thus sacrifices braking distance.
Furthermore, for example, in Japanese Unexamined Patent Publication JP-A-1-208256, there have been proposed an advanced control, in which a target slip ratio of the rear wheels is lowered at the occurrence of unstable condition with high yawing rate, and conversely, a target slip ratio of the front wheels is lowered at the occurrence of insufficient controllability of steering to increase the cornering force.
On the other hand, in Japanese Unexamined Patent Publication JP-A-63-106168, there is proposed a system for setting the target wheel slippage for left and right wheels at the split .mu. road.
However, the above-mentioned prior art sacrifices either the braking efficiency of the brake system or the stability of the vehicle and has not been achieved both of the braking efficiency and the vehicular stability. Namely, control is selected to give importance for the braking performance or the stability performance depending upon the vehicular braking condition.
Even when control is performed giving importance for the stability, it has not been performed the action for canceling unbalance of the left and right braking force on the split .mu. road, in which the friction coefficients at the left and right wheels are difference. Therefore, such control merely reduce the magnitude of unbalance in the braking force.
Therefore, it is an object of the present to provide an anti-skid control system which fundamentally cancels the unbalance of the braking force even on the split .mu. road and thus achieves both of the stability and braking efficiency even on the split .mu. road.